Cycles Render CPU Usage Low with Dual Xeon
Your threads exceed the tiles number (cool!).

You can't render a tile with more then one hyper core thread. Change the tiles size



See this, I got 512x512 tile size with Full HD size rendering, there are 8 tiles been rendering, with 100% CPU in i7-4770k(4C8T).



But with 1024x1024 tiles, there are only 4 tiles to render. So 4 Thread are used with 70% CPU load(Hpyer thread)